The increase in bagworm populations noticed on arborvitae, juniper, red cedar and other trees in southern and central Iowa over the past few years appears to be continuing. Growing populations have also been found in southern and eastern Nebraska. Over 128 plant species have been reported as host plants for bagworm, though arborvitae, red cedar and other juniper species are most commonly attacked.

Large populations of "miniature" bagworms were reported this week from Iowa City. Miniature bagworms, of course, are early instar caterpillars and the small bags that they construct are make of silk and bits of plant needles. Bags collected were about one-half inch in length. By the end of the summer, fully-grown bags will be approximately 2 inches long.

Each bagworm caterpillar produces an individual protective silken bag or case around its body. With its head and legs free, it is able to move about the plant as it feeds on the foliage. Attacked plants usually are partially defoliated, weakened and rendered unsightly. It is not uncommon for complete defoliation to occur in the course of a summer.

If you have had bagworm infestations in the past, now is the time to carefully insect trees and shrubs for small bags. Even if you handpicked last summer's bags during the winter there could still be bagworms present. On a limited number of small trees if may be possible to handpick the entire population while the bags are small.

If handpicking is not practical infested plants should be sprayed as soon as the eggs hatch and small larvae begin feeding. Chemical control becomes less effective as the season progresses because of the increased size of the larva and its bag. Insecticides that can be used against the bagworm include BT (Bacillus thuringiensis), Sevin, permethrin and other pyrethroids, Orthene, or malathion. Apply according to label directions.

Adapted from Iowa State University Horticulture & Home Pest News. June 20, 2007.

It's that time of year again when we begin to see oak wilt affecting our trees. Symptoms usually begin in late spring or early summer and include wilting, necrosis, marginal browning of leaves, and clusters of dead leaves and branches. Oak wilt is caused by the fungus, Ceratocystis fagacearum and infects many oak species including red, pin, white, and bur. Red and pin oaks are more susceptible to oak wilt than white and bur oaks, and can die within 4-6 weeks. White oak species typically develop symptoms more slowly and can die between 1-20 years after infection. This disease is spread by beetle vectors or root grafts, which transmit fungal spores from infected trees to healthy ones.

Marginal browning of red oak leaves.

Clusters of necrotic and dead leaves on red oak.

The oak wilt fungus invades the water conducting tissues of the tree. As a defense mechanism, the plant attempts to stop the pathogen by creating a blockage with tyloses. Tyloses are outgrowths from cells adjacent to the xylem vessels. Once in the xylem, fungal spores are reproduced and travel throughout the vessel system. Spores usually become embedded within the xylem vessels where they produce mycelium or long, filamentous cells, which can spread between xylem vessels. This growth chemically signals the plant that infection has occurred and the production of tyloses begins. Tyloses block the xylem thereby reducing and preventing water movement throughout the tree. The reduction of water movement causes foliage to wilt and eventually causes tree death. The discoloring from dead cells and the blockage produced in the xylem creates the appearance of dark rings when the branch is cut in a cross section. If the bark is peeled back streaks can be seen. Because oak wilt is often confused with other disorders, positive identification requires recovery of the causal fungus from the tree by culturing pieces of wood.

The oak wilt pathogen has a unique disease cycle because it can be spread by both beetles and root grafts. Sap beetles travel from infected to healthy trees, depositing oak wilt spores through wounds in the tree. In addition, when trees are in close proximity to each other their roots graft together. In this manner, the fungus can spread from tree to tree through the water conducting vessels in the roots.

Avoid wounding or pruning oaks during the growing season since sap attracts the beetles that carry the oak wilt fungus. Severing root grafts connecting infected and healthy trees up to 50 feet apart can be used to prevent spread. Fungicide injections are now available to protect healthy trees from the disease. Infected trees can also be treated, but a tree with more than 20% crown loss has little chance for survival.

For more information regarding oak wilt symptoms and control, see ISU Extension publication SUL15, available online.

Grapevines can be supported and trained to a variety of structures. In the home garden, structures range from the decorative arbor to the conventional trellis.

Construction of a grape trellis is similar to constructing a farm fence. The trellis must be substantial enough to carry the weight of the vines plus a heavy crop during high winds. Basically, the trellis consists of two or three wires, one above the other, stretched tightly and secured to firmly-set posts.

End posts serve as the anchor points as well as wire supports. End posts are generally 8 feet long, with a diameter of 4 inches, set approximately 2 feet deep in the soil. They may be braced in several ways. A common method is to set an extra post within a few feet of the end post. A heavy piece of wood or another post makes a good brace between the two end posts. Line posts are also about 8 feet long, but with a diameter of 3 inches. They are set approximately 2 feet into the ground and spaced about 24 feet apart within the row.

Use galvanized wire for the grape trellis. Galvanized wire is durable and does not cause serious wire chafing of young vines. Wire sizes commonly used include numbers 9, 10, or 11. Wires are secured to end posts in various ways. A common method is to wind the wire around the post once or twice and then twist the end several times around the wire as it is stretched to the next post. Some gardeners use special devices to attach the wires to the end posts because they simplify tightening of the wires. These devices employ cranks that eliminate removing the wires from the end posts when tightening. Wires are fastened to the line posts with ordinary staples. Space the wires vertically according to the training system to be followed. For example, a 4-cane-Kniffin system would use 2 wires. One wire should be 3 feet above the ground and the second wire 6 feet off the ground. The 6-cane-Kniffin system uses 3 wires positioned 2, 4, and 6 feet above the ground.

The best time to construct a grape trellis is during the first growing season. Tying new shoots to the trellis wires allows for straight grapevine trunk development in future years.